The powers not delegated to the United States by the Constitution, nor prohibitied by it to the States, are reserved to the States respectively , or to the people. The 10th AmendmentThe powers prohibited to the states are identified in Article I, Section 10 of the U.S. Constitution. Secession is not among them.
The power to prohibit secession is not listed among the powers delegated to the United States.
Secession is therefore a power reserved to the States respectively, or to the people.